The present invention relates to CCD (charge-coupled-device) solid state image sensors in general, and in particular to CCD image sensors with two horizontal transfer CCDs, one of which is located at the upper side of a sensing element array and the other is located at the lower side of the sensing element array. Two polysilicon electrodes are located in each row of the array, arranged in an up position in even columns and in a down position in odd columns, while the other is arranged in an up position in odd columns and in a down position in even columns.
In CCD solid state image sensors of the prior art, attempts have been made to achieve a higher packing density of sensing elements in order to provide a high-definition picture. However, in such devices, which employ horizontal transfer CCDs with a higher driving frequency, the driving circuit is insufficient, reducing the transfer efficiency of the image sensor. Therefore, it is necessary to provide two horizontal transfer CCDs below the vertical transfer channels.
The configuration of a CCD image sensor of the prior art is shown in a plan view in FIG. 1. In this Figure, a multiplicity of photo-sensors 1 acting as light sensing elements are arranged in a multiplicity of columns and rows, and a multiplicity of vertical transfer channels 8 acting as transfer elements are positioned between columns of the photosensors 1.
Signal charges are produced at the light-sensing elements by a photoelectric process as a result of exposure to light from an image.
The signal charges produced in each sensing element are simultaneously transferred to the adjacent transfer channels 8 during the vertical scanning period at the standard television rate. The vertical transfer channels 8 comprise polysilicon electrodes 2 and 3. A predetermined clock pulse is applied as a driving pulse to the electrodes 2 and 3 to move the signal charges formed in the sensing elements one row at a time in the downward direction onto first and second horizontal CCDs 5 and 6. The transferred signal charges are then transmitted in sequence from the first and second horizontal CCDs 5 and 6 to the first and second output terminals, respectively, through each amplifier 7.
However, such a solid state image sensor of the prior art requires a third polysilicon electrode 4 in order to transfer the signal charges between the horizontal CCDs 5 and 6. The need for the three polysilicon electrodes 2 and 3 and 4 arranged in a three-level structure creates a disadvantage because the manufacturing process is complicated. An additional disadvantage of this prior art sensor is that it requires separate driving pulses in order to drive the transfer electrode 4.
Thus there is a need for a CCD image sensor which can be manufactured by a simple process and which requires fewer driving pulses.